This invention relates generally to an electro hydraulic control system and method and, more particularly, to a system and method of pressure compensation for electro hydraulic control systems.
Hydraulic systems are particularly useful in applications requiring a significant power transfer and are extremely reliable in harsh environments, for example, in construction and industrial work places. Earthmoving machines or xe2x80x9cwork machinesxe2x80x9d, such as excavators, backhoe loaders, and front shovel loaders are a few examples where the large power output and reliability of hydraulic systems are desirable.
Typically, a diesel or internal combustion engine drives the hydraulic system. The hydraulic system, in turn, delivers power to operate the machine""s work implement. The hydraulic system typically includes a pump for supplying pressurized hydraulic fluid and a directional valve for controlling the flow of hydraulic fluid to a hydraulically actuated device such as an actuator, cylinder, or motor which in turn delivers power to the work implement, i.e. a bucket. For example, a typical front shovel loader has three basic implement circuits including a boom, stick, and bucket appendages. Individual directional valves and hydraulic cylinders control each appendage. An operator may control the flow of hydraulic fluid, and therefore the velocity of each appendage, through one or more control handles which may be mechanical, electrical or electrohydraulic devices. The control handles provide devices for manual operation, in which the displacement of the control handle is indicative of the desired movement of the associated implement and therefore is also indicative of the flow of hydraulic fluid.
Fluctuations in pressure and flow of the hydraulic fluid supplied to the actuators are inherent characteristics of hydraulic systems. These fluctuations present several problems that the control system must accommodate. Supply pressure fluctuations have several causes. For example, hydraulic circuits are often connected in parallel and are driven by the same pump. Each hydraulic circuit, through its individual operations and load conditions, affects the hydraulic supply pressure. Also, a varying load on the work implement affects the actuator pressure and furthermore affects the amount of flow needed to produce the desired actuator velocity. For example, the work implement may be empty or may be filled and the load may vary while the work implement is moving.
In order to have consistent system response, it is desired to have a fixed flow of hydraulic fluid to move the actuator for a fixed velocity request. Supply pressure variations and varying loads affect the flow rate and therefore, cause the control system to produce undesirable behavior. In particular, it significantly decreases an operator""s ability to accurately control the work implement. This lack of control also causes unnecessary wear and tear on the work implement itself, thereby reducing its effectiveness, further shortening its life span, and increasing the overall costs for maintaining the work machine.
U.S. Pat No. 4,586,332, issued to Schexnayder on May 6, 1986, discloses a two spool valve design for providing pressure compensation. As shown in FIG. 1, a directional control spool 24 has extend, retract and neutral positions for controlling the flow of hydraulic fluid to a hydraulic motor 20. A flow control spool 26 maintains a predetermined pressure differential across the directional control spool 24. Excess fluid from the pump is bypassed by the flow control spool to tank. This two spool valve design attempts to give a fixed flow rate for the extend and retract positions of the direction control spool 26 regardless of the load. However, the valve design is complex and adds cost to the system. Further, the two-spool valve design does not accommodate over-running cylinder loads.
Some control systems use a flow rate control valve to control the flow of hydraulic fluid to the cylinder and thus control its velocity. In the case of hydro-mechanical implementation, the flow rate valve is composed of a metering valve and a pressure compensator. The pressure compensator is used to insure the pressure drop across the metering valve near a constant, whereas the opening of the metering valve can be varied based upon the different flow rate. In the case of electrohydraulic implementation, pressure or pressure differential sensors are used to detect a pressure drop across a valve orifice and the orifice opening is determined by a controller, such as a microprocessor, based upon both pressure drop and desired flow rate. Pressure sensors and pressure differential sensors, however, are expensive. Moreover, they are subject to wear and tear, which significantly decreases their reliability over time, and as a result, they cannot provide a reliable long-term solution to the pressure fluctuations inherent in such hydraulic systems.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the present invention, a method for controlling pressure fluctuations in an electro hydraulic implemented-work element being operated through the use of operator input control mechanism generating operator input signals upon the application thereof, is disclosed. The work element includes an actuator device coupled thereto for controlling the operation thereof. The method comprises the steps of determining a desired and actual velocity of the actuator device, comparing the desired and actual velocity, generating a comparator output signal indicative of a difference between the compared desired and actual velocity, calculating a pressure compensator coefficient representing a ratio between the actual and desired velocity, modifying the comparator output signal by the pressure compensator coefficient to produce an input flow velocity control signal, and inputting the input flow velocity control signal to the valve.